doc: board: Add Intel Edison board doc

This extracts Intel Edison board specific information from
README.x86, converts plain text documentation to reST format and
adds it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
3 files changed